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Giant waves twisting in

solar atmosphere

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 23 March, 2009

For the first time scientists have detected giant waves twisting in the Sun’s lower atmosphere, shedding light on the mystery of why the solar corona is hotter than the Sun’s visible surface.

The Sun’s corona extends more than one million kilometers from its surface and has a temperature exceeding a million degrees celsius, compared with 6,000 degrees at its photosphere. How and why the temperature increases in this way has puzzled scientists for decades.

Scientists have long puzzled why the solar corona is hotter than the Sun's surface. Image: NASA.

“Understanding solar activity and its influence on the Earth’s climate is of paramount importance for human kind,” says Professor Mathioudakis, leader of the Queen’s University Belfast Solar Group. “The Sun is not as ‘quiet’ as many people think. The solar corona, visible from Earth only during a total solar eclipse, is a very dynamic environment which can erupt suddenly, releasing more energy than 10 billion atomic bombs. Our study makes a major advancement in the understanding of how the million degree corona manages to achieve this feat.”

The new research was conducted by scientists from the University of Sheffield and California State University Northridge, as well as Queen’s University Belfast, using the Swedish Solar Telescope in the Canary Islands. The international team uncovered massive solar twists, known as Alfven waves, in the Sun’s lower atmosphere, that hold the key to the Sun’s curious heating mechanism.

Alfven waves result from the twisting of structures in the Sun’s
atmosphere that can be detected by the periodic velocity signals
emitted. In the latest study, the Alfven waves observed were found to be associated with a large magnetic field concentration on the surface of the Sun, approximately twice the size of the British Isles. Unique magnetic oscillations were seen to spread upwards from the solar surface to the Sun’s corona with an average speed of over 20 kilometre per second, carrying enough energy to heat the plasma to well over a few million degrees. These strong magnetic fields manifest as bright features, often with lifetimes exceeding one hour.

Alfven waves were inferred by studying magnetic field oscilliations powering away from the Sun's surface to the corona. Image: QUB.

“Often, waves can be visualized by the rippling of water when a stone is dropped into a pond, or by the motions of a guitar string when plucked,” says Dr David Jess of Queen’s University Belfast and lead author of the paper that appears in this week’s issue of the journal Science. “However, Alfven waves cannot be seen so easily. In fact, they are completely invisible to the naked eye. Only by examining the motions of structures and their corresponding velocities in the Sun’s turbulent atmosphere could we find, for the first time, the presence of these elusive Alfven waves.”

Now that these elusive waves have been detected, they can be used to determine the physical conditions in the invisible regions of the Sun and other stars, through the technique of magneto-seismology. “It was a real thrilling experience to interpret the data found by my colleagues at Queen’s University,” says Professor Erdelyi, who lead the theoretical interpretation of the Alfven waves.

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